Oil wells eventually become depleted and little more can be pumped from them. Production drops to a marginal amount or below that. Under these circumstances, it is conventional to withdraw such equipment as is salvageable and then to plug the well and abandon it.
The decrease in production may be due to actual substantial depletion of the oil in the oil bearing stratum or strata or to plugging of the perforations of the liner through which oil is drawn into the tubing and pumped from the well. Sometimes, steam has been forced down through the tubing and liner, and production has then increased for a while and then decreased again. The additional production has been worth the expense involved, even though the oil production may have been increased only for a month or two.
I have discovered a new method for increasing oil production from old oil wells that appear to be substantially exhausted, even after steam treatment has been tried. This method may not work for every type of oil well or for every type of oil bearing strata, but it has succeeded under certain circumstances and may well be applicable to others.
The invention, as discussed below uses particular techniques involving cyclic vibration. Such techniques are to be differentiated from the different techniques recommended by various early patents, mainly those of Albert G. Bodine. Mr. Bodine has been active in proposing the stimulation of wells by vibration for quite a long time; his first U.S. Pat. No. 2,437,456 was applied for in 1941. Bodine U.S. Pat. No. Re. 23,381 of 1951 cites 50 Hertz as an appropriate frequency. He also proposed vibrating the tubing with the liner attached and proposed that adjacent wells can be stimulated.
Bodine's U.S. Pat. No. 2,667,932 of 1954 included the use of counter-rotating masses for the excitation of the pipe string. Also Bodine proposed to anchor the bottom of the pipe to the oil bearing region. His patent states that exciting the bottom of a casing which has been cut just above the oil bearing region is one way to couple the surface-produced vibration to the oil bearing region. He also appreciated that "The means for generating these vibrations may be mechanical, electrical, hydraulic . . . "; an effective frequency range is stated to be 10 to 30 Hertz. He also appreciated that "a column having heavy mass and large area in contact with the formation" (liner) is important to the efficient transmission of energy to the formation. He alludes to, but does not include in his claims, the importance of the column of fluid in the well. This column helps to overcome the pressure caused by the overburden.
Bodine's U.S. Pat. No. 2,680,485, 1954, employed a vibrator at the surface attached to tubing which was in turn firmly attached to a perforated liner at the bottom of the well. This whole system was excited in tension and compression by the surface vibrator.
Bodine's U.S. Pat. No. 2,700,422, 1955, seems to embody the disclosure of all of his previous patents concerning stimulation and pumping of oil wells.
Bodine's U.S. Pat. No. 2,871,943, 1959, describes a down hole vibrator which was claimed to be powerful enough to fracture the formation and decrease the permeability, as opposed to merely stimulating oil production.
His U.S. Pat. No. 3,016,093; 1062, describes the generation of asymmetrical pressure waves, as opposed to the sinusoidal type of waves that the present invention produces.
Resonant dynamic excitation offers significant advantages. However, in a system which is controlled by the power input (e.g., the rotational speed of an engine), a potential "runaway" situation exists, for when the maximum power input for a particular resonance is exceeded, the engine may speed up greatly, because the pipe can absorb less power at a frequency higher than resonance. This problem will be explained below in more detail.
Another potential problem is that of exciting harmful modes of vibration of the derrick. Modes of vibration which have a lower resonant frequency than the desired mode and which involve different parts of the derrick and support structure, have large and potentially harmful vibrational amplitudes. A system which increases the operating frequency to arrive at the desired mode tends to excite these harmful modes and create hazardous conditions.
Among the objects of the invention are these: to provide a practical and economic method for stimulating oil production; to keep the drill pipe or string at resonance when and if the resonant frequency changes; to provide for relatively low power operation; and to provide controls that protect the apparatus from damaging itself.